The germicidal, disinfecting effect of UV radiation has been known for a long time. UV lamps have been used for several decades for disinfecting drinking water and wastewater, in air conditioning systems, sumps and for disinfecting work areas in biological laboratories. UV radiation is produced when disinfecting water and is released into the water thus reaching germs (viruses, bacteria, protozoa). The germicidal effect of the UV radiation is dependent here on the wavelength and on the type of microorganisms that are to be inactivated.
Ultraviolet radiation or ultraviolet light is described as the wavelength range of the electromagnetic radiation, which is between visible light and x-ray radiation, i.e. the wavelength range between 400 nm and 100 nm. The entire UV spectrum is typically divided into 4 areas here, namely UV-A (315 nm-400 nm), UV-B (280 nm-315 nm), UV-C (200 nm-280 nm) and vacuum UV or V-UV (100 nm-200 nm). The germicidal, disinfecting effect of UV radiation is achieved mainly with the wavelength ranges of UV-B and UB-C. The germicidal effect of UV-A is relatively small compared with UV-B and UV-C.
Virtually all water disinfecting systems are operated using UV lamps, which are configured as gas discharge lamps with a mercury content in the gas filling. Mercury produces inter alia a dominant emission curve at 254 nm, which is close to a maximum of the wavelength-dependent effectiveness of UV light for the disinfection of microorganisms. The dependence of effectiveness on the wavelength has a local maximum of 260 nm in most microorganisms, i.e. a specific dose of UV radiation of this wavelength is particularly effective. Initially, the effectiveness drops to 240 nm towards the shorter wavelengths and then increases again. The range between 240 nm and 200 nm with a good level of effectiveness is also suitable for disinfecting microorganisms.
Two types of lamps are used in the main, so-called low pressure lamps which are operated at a gas pressure of less than 0.1 mbar. These low pressure lamps have an extremely narrow-band curve spectrum and in the cited wavelength range almost exclusively emit UV-C radiation with a wavelength of 254 nm. They are characterised by extremely high electrical efficiency, since approximately 40% of the total electrical power consumed is converted into radiated power of the cited wavelength. The disadvantage with low pressure lamps is that the absolute radiated power is relatively low in relation to their overall size and consequently a large number of lamps have to be used in disinfecting systems with a large throughput of water. These systems are correspondingly expensive. Their advantage on the other hand is that the radiation output only has to be monitored at 254 nm to control the lamps, since other components only play a minor part in the disinfection activity and the effective curve can therefore be used directly to control the system.
DE 20 2004 012 686 U1 describes a disinfecting device comprising a UV lamp, which is arranged in a piston and the emission thereof is monitored using a sensor that is not described in further detail. In the event of deviation from a target value, a signal is to be given indicating that cleaning is required.
A disinfecting system comprising UV lamps, which is monitored using two sensors, is known from DE 10 2008 051 239 A1. A deterioration and possible attenuation of the overall radiation output is determined by means of a different geometric arrangement of the sensors, for example, by a different distance or a different angle of detection. The Lambert-Beer'sche law is used here to determine any tarnishing of the lamp casings caused by deposits. This document contains no indication of the spectral sensitivity of the UV sensors. Neither does it state that both the UV sensors may have different spectral sensitivities.
Other water disinfecting systems use so-called medium pressure lamps. The internal pressure of said lamps is between approx. 0.1 and 10 bar. The lamps are operated at higher temperatures and with considerably more compact dimensions have much greater power consumption and correspondingly higher UV radiation output. The higher temperatures and the higher pressure inside the lamp initiate other UV-C curves and continuum radiation between 240 nm and 200 nm. As stated above, this range is also relevant in terms of disinfection performance since the effect of a given UV dose on microorganisms in this wavelength range is also great.
To date, the monitoring of medium pressure sensors has been similar to the monitoring and control of the radiation output of low pressure lamps. UV sensors are used for this purpose that cover part or the whole of the spectrum. The performance of the medium pressure lamp is then controlled such that the total emission registered by the sensor corresponds to requirements and specifications. A spectrum is thus covered in the prior art which is dominated by curves and a continuum with wavelengths of more than 240 nm. An ultraviolet sensor for said monitoring is described in US 2004/0200975 A1, for example. An SiC sensor is disclosed here which has maximum sensitivity at approximately 260 nm.